Acid photogenerators, per se, are known as are their use in photoresist imaging elements. Acid photogenerators are disclosed, for example, in U.S. Pat. Nos. 4,081,276; 4,058,401; 4,026,705; 2,807,648; 4,069,055 and 4,529,490. In recently issued U.S. Pat. No. 4,661,429 to Molaire, et al., there is disclosed a photoelectrographic element for use in a photoelectrographic process which comprises a conductive layer in electrical contact with an acid photogenerating layer which (a) is free of photopolymerizable materials and (b) comprises an electrically insulating binder and an acid photogenerator. The photoelectrographic process disclosed therein comprises the steps of:
(a) providing a photoelectrographic element comprising a conductive layer in electrical contact with an acid photogenerating layer which (i) is free of photopolymerizable materials and (ii) comprises an electrically insulating binder and an acid photogenerator;
(b) carrying out the following steps (b)(i) and (b)(ii) concurrently or separately in any order to form an electrostatic image,
(i) imagewise exposing the acid photogenerating layer to actinic radiation, PA0 (ii) electrostatically charging the acid photogenerating layer, and
(c) developing the electrostatic latent image with charged toner particles.
The imaging technique or method disclosed by Molaire, et al., takes advantage of the fact that exposure of the acid generator significantly increases the charge decay of the electrostatic charges in the exposed area of the layer. Imagewise irradiation of the acid photogenerator layer creates differential charge decay between exposed and unexposed areas. When imagewise irradiation is coupled with the step of electrostatic charging, this differential charge decay or imagewise conductivity differential forms or creates an electrostatic latent image. The latent image is developed by contacting the photoelectrographic layer with a charged toner composition of the type used in electrophotographic development operations. Such toner compositions are well known, being described in numerous patents and other literature such as U.S. Pat. Nos. 2,296,691; 4,546,060; 4,076,857 and 3,893,935. In the Molaire, et al., process, exposure can occur before, after or simultaneously with the charging step. This is different from electrophotographic imaging techniques where the electrophotographic element must always be charged electrostatically prior to exposure.
The photoelectrographic elements of Molaire, et al., also are advantageous in that the imagewise differential charge decay of electrostatic charges are erasable with heat. In addition, the imagewise conductivity differential created by the exposure is permanent unless the element is subjected to heat. Thus, multiple copies of a document can be made from a single exposure. Further, the photoelectrographic layer can be developed with a charged toner having the same polarity as the latent electrostatic image or with a charged toner having a different polarity from the latent electrostatic image. In one case, a positive image is formed. In the other case, a negative image is formed. Alternatively, the photoelectrographic layer can be charged either positively or negatively, and the resulting electrostatic latent images can be developed with a toner of given polarity to yield either a positively or negatively toned image. According to Molaire, et al., any compound which generates an acid upon exposure can be used in the photoelectrographic element. However, aromatic onium salts, including triarylselenonium salts and aryldiazonium salts, and 6-substituted-2,4-bis(trichloromethyl)-5-triazines are especially preferred.
While the photoelectrographic elements of Molaire, et al., constitute a significant contribution to the art, they suffer from the disadvantage that they are sensitive to variations in the moisture content of the surrounding atmosphere. That is, as the relative humidity in the surrounding atmosphere increases, the photoelectrographic elements of Molaire, et al., become more conductive. Conversely, as the relative humidity in the surrounding atmosphere decreases, they become less conductive and more insulating. This change in conductivity is observed for both the exposed and unexposed regions or areas of the photoelectrographic element to differing extents depending upon the specific formulation of the element. For example, in certain instances, under high relative humidity conditions, the unexposed area of a particular element may not be capable of supporting a charge high enough to create a potential difference between the exposed and unexposed areas which is sufficient to yield a toned image of acceptable contrast. That is, either the D.sub.max areas are much lower in density than desired or the D.sub.min areas are darker than desired. Conversely, in other photoelectrographic elements of different formulations, under low relative humidity conditions, the exposed areas of the element may only discharge to a level which is insufficiently lower than the level retained on the unexposed areas of the element. Again, the difference in potential available for toning is too small to yield images of acceptable contrast and quality. Furthermore, while a given formulation may perform adequately at a given relative humidity, its electrical performance may change significantly in response to changes in relative humidity such that image quality becomes unacceptable. Such a formulation would not be generally useful is widely varying climates around the world.
In addition, the photoelectrographic elements of Molaire, et al., suffer from other disadvantages in that certain of the binders used by Molaire, et al., in the acid photogenerating layer exhibit undesirable defects, such as poor adhesion to the conducting or barrier layers used in the element, as in the case of certain of the polycarbonates such as bisphenol-A, and other defects, such as brittleness or crazing, which precludes the element or film from being used in the form of a drum, as in the case of poly(vinyl phenol), for example, which requires a flexible film that will not crack when it is bent or wrapped around a cylinder.
Accordingly, it would be highly desirable to be able to provide a photoelectrographic element of the type described by Molaire, et al., which not only possesses all of the desirable above-mentioned properties and features but, in addition, one which is substantially insensitive to widely varying changes in relative humidity which are encountered during normal operating conditions so that both charge acceptance and the persistent photo-induced conductivity remain within the range required for high quality imaging. Further, it would also be highly desirable to provide such an element which is free of the above-mentioned defects such as poor adhesion, brittleness and crazing. The present invention provides such a photoelectrographic element and a method of forming images with the element.